The present invention relates to a process for scrubbing exhaust gases from diesel engines using filter cartridges of open pore ceramic foam, and relates too to a filter cartridge for carrying out the process.
The soot content of waste gases emitted by diesel engines can, basically, be reduced in two ways viz., by optimising the preparation of the mixture and the combustion sequence in the engine, and by inserting filters in the path of the exhaust gas stream. Depending on the design of the filter, the amount of soot extracted is usually 50-90%.
The active filter volume, however, cannot be freely changed in order to achieve optimum extraction of soot, because the filter constitutes a resistance that creates a back-pressure acting on the exhaust gas. This back-pressure must not be too high, otherwise the power output of the engine will be reduced and the fuel consumption increased. In the case of diesel engines in private cars fitted with exhaust gas filters, the back pressure should at most be 0.2 bar.
As the exhaust gas filter would quickly become blocked with soot, the latter must be burnt off from time to time. The interval from one regeneration phase to the next regeneration phase is designated the regeneration interval. Under certain driving conditions the filter regenerates itself automatically. The soot deposit is then combusted at temperatures of 550.degree.-700.degree. C., if the filter features a catalyt coating, the combustion temperature can be lowered to about 400.degree. C.
Irregular regeneration can lead to uncontrolled combustion causing much higher peak temperatures due to accumulation of heat. Because the high peak temperatures that occur during the combustion of the soot are followed by cooling to the exhaust gas temperature, it is necessary to employ filter materials that are capable of withstanding high temperatures and thermal shock.
From the U.S. Pat. No. 4,264,346 it is known, for the above mentioned purpose, to employ an open pore ceramic foam of cordierite in the form of a solid cylinder. By an open pore ceramic foam is to be understood here a porous ceramic body with a three dimensional network and cellular structure containing a plurality of continuous, interconnecting hollow spaces. The ceramic body is situated in the path of the exhaust gas stream of the diesel engine in such a manner that the exhaust gases strike the ceramic body end-on and flow through it parallel to its central axis. The deposit of soot is greatest in that part of the filter through which the gases flow first and decreases rapidly with increasing passage of exhaust gas. As a result the part of the filter with which the gases have first contact reaches its soot capture capacity after a relatively short time and has to be regenerated; the rest of the filter is still capable of capturing soot but is then likewise subjected to the regeneration process earlier that in fact is necessary.
Although cordierite ceramic is particularly resistant to temperature changes, the handling of a filter of the kind described above leads relatively quickly to mechanical damage of the filter because of the relatively frequent regeneration required. A further disadvantage of such a filter is that filtration is not uniformly efficient over the whole length, and the less efficient part of the ceramic filter contributes substantially to the back pressure mentioned at the start.
The object of the present invention is to eliminate at least in part the above mentioned disadvantages encountered in the scrubbing of diesel engine exhaust gases using filters of open pore ceramic foam.